Use of impingement heating apparatus for thermally treating food products, such as in cooking or thawing, has been disclosed in the prior art. These conventional apparatuses typically employ columniating jets of temperature-controlled gas which impinge against the surface of a food product moving relatively thereto. Such devices are disclosed, for example, in U.S. Pat. Nos. 4,525,391, 4,338,911 and 4,154,861. When used for cooking purposes, certain of these devices are employed in combination with microwave generators for the apparent reason that impingement cooking, by itself, is not entirely satisfactory and needs supplementation from other cooking equipment.
In conventional conveyorized ovens which utilize impingement eating, high velocity jets of a temperature-controlled gas are directed against the surface of food products transported through the oven on a conveyor. The temperature controlled gas, or cooking vapor, is discharged from a blower or fan into a plenum or enclosed duct that directs the flow of vapor into a series of spaced-apart ducts extending transversely across the conveyor. These ducts are in turn adapted to direct the flow of gas into columniating orifices which causes the gas to impinge against the surface of the food products. These ducts are difficult to clean and require extra effort in the clean up operation with close attention to the details of complete cleaning required in the process food industry. Moreover, where the food products treated in the oven have a substantial fat content which will be rendered out as liquid fat it is highly desirable to recover the fat as a valuable by-product. Bacon processing companies sell such recovered fat to other industries such as for use in animal feed. Further in the oven is necessary to control the liquid fats so as to prevent combusting the fats and creating a smoke atmosphere in the oven causing an "off" taste to the main product.
Furthermore, those working with such ovens in the past have encountered difficulty in balancing the vapor flow across the plenum and into the various ducts, especially those farthest removed from the point at which the cooking vapor is discharged from the blower. One result is that food products, from side to side on the conveyor belt, are exposed to the cooking vapor at uneven rates resulting in certain products which are fully cooked, others overcooked and yet others substantially undercooked. This is typical where the gross cooking vapor flow is transversely to the movement of the product. Non uniformity is another result with the color variations indicating uneven cooking of the product from one side of the belt to the other. This is apparent when viewing the loaded conveyor belt. Such a situation gives an unacceptable standard for application in high quality commercial production operations and often leads to a great deal of wasted product rejected by the quality control department. Undercooked meats such as hamburger patties can carry live bacteria including E-coli which are very unsafe to human health.
Plenums or cooking air ducts positioned on the outside of the conventional commercially available impingement ovens are fed from a blower or fan typically arranged inside a shroud at the side of the principal air delivery plenum or enclosed duct. These ducts collect dirt such as grease, crumbs and other deposits from the cooking operations which must be cleaned out periodically. And moreover, in the use of the single blower assembly, consisting typically of a fan wheel and a fan drive motor, attempts have been made to balance the flow of cooking air into the distribution ducts farthest removed from the blower by tapering the walls, thus diminishing the cross-sectional area of the ducts. This is not entirely effective. In an attempt to balance or control air flow as between the streams directed to the top and to the bottom of the product, dampers are often employed in the external, enclosed ducts. Although dampers in the air distribution system may serve somewhat the objective of air flow balance, they cannot either increase or decrease the overall mass flow in the oven. The single blower is the limiting factor.
It has been found in the prior art that the interior oven surfaces between the air discharge orifices and the blower or fan intake cause turbulence within the cooking chamber. This further disrupts the return flow of the treatment vapor and even hinders the efficient heat distribution of the treatment vapor emitted from the orifices onto the product. It will be understood that after the cooking gases have issued from the orifices and impinged upon the product, ideally the gases should be removed as efficiently as possible from the vicinity of the product in order that the cooking gas' following behind will be permitted to engage the product and not be obstructed by stagnant or disordered circulation zones created within the oven proper. That situation or condition is undesirable from the standpoint of achieving efficient heat transfer from the circulating treatment air onto the products being cooked.
Following a completed cooking cycle the oven is cleaned and in many oven models this is a labor intensive process which absorbs considerable non operating or down time for the oven. Vapors and juices created in the cooking process frequently are deposited on the interior walls of the oven as well as in the vapor distribution ducts and require an opening up of the oven for cleaning and visual inspection. A highly desirable feature in an oven is the ability to clean the unit without necessarily opening up to expose the oven interior and with a highly reduces reliance on manual cleaning.
In view of these and other disadvantages that have been encountered in using the conventional, commercially available oven apparatus, an improved duct-less impingement oven is needed that will facilitate even distribution of the food treatment vapors across the conveyor belt for more even heat application to the products carried thereon, an oven which will afford substantially reduced turbulence therein, and wherein the return flow of the cooking vapors to the fan for the reticulation cycle will be within the oven proper and not through the hard to clean, usually invisible, interior surfaces of outside plenums or ducts.